If you want the exact math:
Federer is defending USO Win (1000) points these 2 weeks
Roddick is defending USO 1st round (5) points these 2 weeks
If Federer defeats roddick in the finals, (wont happen, Roddick is going down in the semis
) then Federer keeps all his points, and doesnt gain any. Roddick on the other hand, was only defending 5 points from last year. He loses those 5, true, but picks up 700 for his finals run.